Diffractive optical element fabrication

ABSTRACT

Described herein are embodiments of a diffractive optical element (23) such as a grism. In one embodiment, the diffractive optical element (23) includes an input surface (31) configured to receive an input optical signal (29), a diffractive surface (33) adapted to spatially disperse the input optical beam (29) into a dispersed signal and an output surface (35) configured to output the dispersed signal from the diffractive optical element. The input surface (31) and the diffractive surface (33) are non-parallel and the diffractive surface (33) is formed in situ by a photolithographic technique.

The present application is a non-provisional of copending U.S.Provisional Patent Application Ser. No. 62/657,739, filed Apr. 14, 2018,and entitled “DIFFRACTIVE OPTICAL ELEMENT FABRICATION”. The entirecontents of U.S. Patent Application Ser. No. 62/657,739 are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to diffractive optical elements andmethods for their manufacture. Embodiments of the invention have beenparticularly adapted for optical grating-prisms (or “grisms”). Whilesome embodiments will be described herein with particular reference tothat application, it will be appreciated that the invention is notlimited to such a field of use, and is applicable in broader contexts.

BACKGROUND

Any discussion of the background art throughout the specification shouldin no way be considered as an admission that such art is widely known orforms part of common general knowledge in the field.

In optical devices, a grism is a diffractive optical element comprisinga combination of a prism and a diffraction grating. Grisms are used in asimilar manner to diffraction gratings to select particular wavelengthsof light but provide increased dispersive power to better separate theconstituent wavelengths from one another.

Current techniques for fabricating high performance grisms includewriting of a diffraction grating onto a glass wafer, and subsequentlybonding that wafer onto a prism with an epoxy or optical contactbonding. This two-step fabrication technique is typically done usingexpensive semiconductor grade steppers (Nikon, etc. . . . ), whichrequire multiple exposures per wafer. Steppers also provide very lowdepth of focus which limits the grating writing to ultra-flat substrate(typically flat to within <0.3 μM) to print high resolution structures.

The inventors have identified some deficiencies in this technique. Forexample, not only does optical component bonding require additionalequipment, optical performance consistency has been found to becompromised with bonded optics.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention there isprovided a diffractive optical element including:

-   -   an input surface for configured to receiving an input optical        signal;    -   a diffractive surface for adapted to spatially dispersing the        input optical beam into a    -   dispersed signal; and    -   an output surface for configured to outputting the dispersed        signal;    -   wherein the input surface and the diffractive surface are        non-parallel and; and    -   wherein the diffractive surface is formed in situ by a        photolithographic technique.

In some embodiments the diffractive surface and output surface arenon-parallel.

In some embodiments the input surface is the output surface.

In some embodiments the optical element is a triangular prism. In someother embodiments the optical element is a trapezoid.

In some embodiments the diffractive surface is formed by:

-   -   depositing a layer of pattern material to the diffractive        surface;    -   applying a photoresist layer to the pattern material layer;    -   creating a diffractive pattern in the photoresist layer using a        light source; and    -   transferring the diffractive pattern in the photoresist layer to        the pattern material.

In some embodiments the diffractive surface is formed by:

-   -   depositing a photoresist layer to the diffractive surface; and    -   creating a diffractive pattern in the photoresist layer using a        light source.

The pattern material may be selected from one or more of Si3N4, TiO2,HfO2, amorphous silicon, high refractive index polymer, reflective metaland Ta2O5.

In some embodiments the high refractive index polymer is spin-on-glassor photoresist.

In some embodiments the reflective metal is selected from chromium,gold, silver, aluminum or nickel.

In some embodiments the photoresist layer includes an anti-reflectivecoating and primer.

In some embodiments the diffractive pattern in the photoresist layer iscreated by illuminating the photoresist layer through a photomask. Insome embodiments the diffractive pattern in the photoresist layer iscreated by illuminating the photoresist layer through a photo mask whilemoving the photoresist layer relative to the mask. In some embodimentsthe movement is substantially continuous. In some embodiments thephotoresist layer is moved a distance of

$z_{T} = \frac{2\; p^{2}}{\lambda}$

where p is the spatial period of the mask and λ is the wavelength of thelight source.

In some embodiments the step of depositing a layer of pattern materialto the diffractive surface includes sputter coating. In some embodimentsthe step of depositing a layer of pattern material to the diffractivesurface includes evaporation. In some embodiments the step of depositinga layer of pattern material to the diffractive surface includes chemicalvapor deposition, plasma-enhanced chemical vapor deposition or lowpressure chemical vapor deposition.

In some embodiments the diffractive optical element is configured tooperate in a wavelength selective switch. In these embodiments the inputoptical signal includes a plurality of optical wavelength channels.

In alternative embodiments, the diffractive optical element includes alens.

In accordance with a second aspect of the present invention there isprovided a diffractive optical element including:

-   -   an input surface for configured to receiving an input optical        signal;    -   a diffractive surface for adapted to spatially dispersing the        input optical beam into a dispersed signal; and    -   an output surface for configured to outputting the dispersed        signal;    -   wherein the output surface and the diffractive surface are        non-parallel; and        -   wherein diffractive surface is formed in situ by a            photolithographic technique.

In some embodiments, the input surface is the diffractive surface.

In accordance with a third aspect of the present invention there isprovided a diffractive optical element including:

-   -   an input surface configured to receive an input optical signal;    -   a diffractive surface adapted to spatially disperse the input        optical beam into a dispersed signal; and    -   an output surface configured to output the dispersed signal;    -   wherein the input surface and the diffractive surface are        separated by a distance of greater than 5 mm; and    -   wherein the diffractive surface is formed in situ by a        photolithographic technique.

In preferred embodiments, the diffractive optical element is non-waferstructure.

In accordance with a fourth aspect of the present invention there isprovided a method of fabricating a diffractive optical element, themethod including the steps:

-   -   depositing a layer of pattern material directly onto a first        surface of a non-cuboid prism, wherein the first surface is non        parallel to either or both of an input surface configured to        receive an input optical signal and an output surface configured        to output a dispersed signal from the diffractive optical        element;    -   applying a photoresist layer to the pattern material layer;    -   creating a diffractive pattern in the photoresist layer; and    -   transferring the diffractive pattern in the photoresist layer to        the pattern material to define a diffractive surface on the        first surface.

In accordance with a fifth aspect of the present invention there isprovided a method of fabricating a diffractive optical element, themethod including the steps:

-   -   depositing a photoresist layer directly onto a first surface of        a non-cuboid prism, wherein the first surface is non parallel to        either or both of an input surface configured to receive an        input optical signal and an output surface configured to output        a dispersed signal from the diffractive optical element; and    -   creating a diffractive pattern in the photoresist layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the disclosure will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is a schematic front view of a diffraction grating;

FIG. 2 is a schematic illustration of optical rays through a grism;

FIG. 3 is a perspective view of a wavelength selective switchincorporating a grism as a diffractive element;

FIG. 4 is a process flow diagram illustrating the primary steps of amethod of fabricating a grism having a diffractive surface that isformed in situ by a photolithographic technique;

FIG. 5 is a schematic side view of a grism being fabricated according tothe method of FIG. 4, the grism being illustrated during an exposureprocess;

FIG. 6 is a schematic side view of a completed grism fabricated by themethod of FIG. 4;

FIG. 7 is a schematic side view of a grism being fabricated according tothe method of FIG. 4, the grism being illustrated during an alternativeexposure process to that of FIG. 5;

FIG. 8 is an illustration of an exemplary diffraction pattern producedby light passing through a mask structure; and

FIG. 9 is an illustration of an integrated diffraction pattern producedby light passing through a mask structure.

DETAILED DESCRIPTION

Embodiments of the invention will be described herein with specificreference to fabricating grism devices for use in wavelength selectiveswitch (WSS) devices. However, the person skilled in the art willappreciate that the principles described herein are applicable to otheroptical systems and devices.

System Overview

Referring to FIG. 1, there is illustrated schematically a diffractiongrating 1 for use in an optical system. The diffraction grating includesa substrate 2 and a substantially linear array of elongate diffractingelements 3 arranged in a grating profile across substrate 2. In general,each diffracting element may include a relative degree of curvatureacross the face of grating 1 as described in US Patent ApplicationPublication 2014/0347733, to Stewart et al., entitled “Systems andMethods of Aberration Correction in Optical Systems” and assigned toFinisar Corporation. However, for the sake of simplicity, all of theelements 3 in FIG. 1 have zero curvature. Typically, diffractingelements are diffraction lines and include grooves or ridges for areflective grating, or slots for a transmissive grating. Alternatively,the diffracting elements may be defined as regions in a material havinga periodic variation in refractive index.

In certain optical systems, such as the one discussed below withreference to FIG. 3, it is beneficial to use a grism to spatiallydisperse wavelengths in an optical beam. A cross-sectional view of anexemplary grism 23 is shown in FIG. 2. The grism 23 includes on opticalprism 25 with a diffraction gating 1 located on one of the faces of theprism 25. It will be appreciated that the triangular form of the prism25 illustrated is exemplary only and in practice could be any prismstructure. In preferred embodiments, the prism is a non-cuboid shape.

Cuboid structures are defined as any rectangular prismatic polyhedron.That is, a six faced polyhedron having three pairs of correspondingparallel faces, wherein the faces of each pair have like dimensions andeach pair of faces are perpendicularly disposed with respect to theother pairs of faces. Examples of cuboid structures include cubes andrectangular prisms. Examples of non-cuboid structures includetrapezoidal prisms, triangular prisms, structures with curved faces andnon-rectangular parallelepipeds (structures having at least onenon-right angled face).

More generally, the embodiments of the invention are advantageous innon-wafer like prisms which have non parallel surfaces and greaterthickness along the optical axis. It is in these non-wafer geometriesthat direct writing of grating structures is difficult. By wafer, whatis meant is a thin slice of substrate, either rectangular, oval orcircular (to form a think disk) but having parallel input and outputsurfaces along the optical axis. Wafers for optical use typically havethicknesses less than 1 mm.

In operation, an input optical beam 29 is incident onto a first facedefining an input surface 31 of grism 23. The beam 29 is refractedthrough the prism 25 and incident onto a second face defining adiffractive surface 33 of grism 23. The diffractive surface 33, which isnon-parallel to the input surface, includes grating 1 and acts todiffract or spatially disperse the optical beam 29 into a diffractedspectrum including a plurality of wavelengths. These wavelengths arediffracted at different angles and are coupled out of prism 25 at athird face defining an output surface 35.

In other embodiments (not illustrated), grism 23 may operate in otherconfigurations in which the optical beam 29 is both input and outputthrough the same surface of prism 25.

Overview of Exemplary WSS Framework

With reference to FIG. 3, a general overview of WSS devices will now bedescribed. FIG. 3 illustrates schematically an exemplary WSS opticalswitching device 4 configured for switching input optical beams fromthree input optical fiber ports 5, 6 and 7 to an output optical fiberport 9. It will be appreciated that device 4 is reconfigurable such thatinput ports 5, 6 and 7 are able to be used as outputs and output port 9used as an input. The optical beams are indicative of WDM opticalsignals, as mentioned above. On a broad functional level, device 4performs a similar switching function to that described in U.S. Pat. No.7,397,980 to Frisken, entitled “Dual-source optical wavelengthprocessor” and assigned to Finisar Corporation, the contents of whichare incorporated herein by way of cross-reference. The optical beamspropagate from input ports 5, 6 and 7 in a forward direction and arereflected from an active switching element in the form of a liquidcrystal on silicon (LCOS) device 11 in a return direction to output port9. In other embodiments, other types of active switching elements areused in place of LCOS device 11, including arrays of individuallycontrollable micro-electromechanical (MEMs) mirrors.

Device 4 includes a wavelength dispersive grism element 13 for spatiallydispersing the individual wavelength channels from an input optical beamin the direction of a first axis (y-axis). It will be appreciated bypersons skilled in the art that the dispersive element is not limited toa grism configuration, but may be any type of diffraction gratingelement. Grism element 13 operates in a manner described in U.S. Pat.No. 7,397,980. That is, to spatially separate the constituent wavelengthchannels contained within each optical beam in the y-axis according towavelength. Grism 13 includes a diffraction grating portion which, inaddition to the spatial diffraction function, also at least partiallycorrects beams for optical aberrations present in device 4.

A lens 15 is positioned adjacent to grism 13 such that the optical beamstraverse the lens both prior to incidence onto grism 13 and afterreflection from the grism. This double pass of lens 15 acts to collimatebeams in the direction of a second axis (x-axis). Similarly, inpropagating between input ports 5, 6 and 7 and LCOS device 11, the beamsreflect twice off a cylindrical mirror 17. Mirror 17 has appropriatecurvature such that each dispersed channel is focused onto the LCOSdevice in the y-axis.

The dispersed wavelength channels are incident onto LCOS device 11,which acts as a reflective spatial light modulator to activelyindependently steer each channel in the x-axis. At the device level,LCOS device 11 operates in a similar manner to that described in U.S.Pat. No. 7,092,599 to Frisken, entitled “Wavelength manipulation systemand method” and assigned to Finisar Corporation, the contents of whichare incorporated herein by way of cross-reference. As mentioned above,in other WSS designs, other types of switching element are used in placeof LCOS device 11, such as micro electromechanical mirror (MEMs) arrays.

Description of Existing Grism Fabrication

Current techniques for fabricating high performance grisms includewriting a diffraction grating onto a substantially planar glass wafer orslide, and subsequently bonding that wafer onto a prism with an epoxy,optical contact bonding or some other suitable bonding method.

The inventors of the subject invention have identified a number ofproblems associated with this two-step technique such as opticalperformance issues, as well as practical considerations with each stepof the manufacturing process.

A major issue identified by the inventors with grisms made according tothe two step process is that optical performance consistency is low. Thesource of the inconsistency has been attributed to mechanical andoptical imperfections, such as foreign material or voids in the bondinterface, delamination of the of the grating and prism, introduction ofa depletion layer or layer with different refractive index or mechanicalstress at the interface. These mechanical and optical imperfections havebeen attributed with introducing artifacts such as wave front distortionand ripple. This low performance consistency results is a reduction inthe performance of the grism and ultimately the entire optical system.

Several practical complications have also been identified with the twostep process. The first of these relates to the initial step of writingthe grating onto the wafer. Practically, this step is performed usingstandard photolithography techniques commonly used in the semiconductorindustry. These techniques require expensive semiconductor gradesteppers, which require multiple exposures per wafer. The steppers alsoprovide very low depth of focus which limits the grating writing toultra-flat wafers or slides (typically <0.3 um) and have correspondinglylow alignment tolerances with the photolithography mask to produce highresolution structures. Furthermore, this equipment is limited tooperating on thin (typically 1 mm or less) slides only. This reliance ontraditional photolithography techniques has been identified as asignificant source of input costs when fabricating grisms due to thecost of the required equipment and the time required to achieve thenecessary degree of alignment precision.

A second practical issue associated with the two step process is thealignment of the grating on the prism during the bonding step. Thisalignment step introduces additional potential for misalignments (inaddition to the alignment of the mask during the first step) which haveto be accounted for. This additional alignment requirement slows downthe manufacturing process and requires additional equipment therebyfurther increasing the cost and complexity of grisms manufactured inthis way.

In Situ Grism Fabrication

To overcome or ameliorate at least some of the issues identified withthe prior art, the inventors have developed an in situ grism fabricationtechnique wherein the diffraction grating is formed directly onto aprism in situ rather than on a separate wafer element which issubsequently fixed to the prism. The basic in situ fabrication method 40is outlined by the flow chart illustrated in FIG. 4 and the resultingstructures are illustrated schematically in FIG. 5. An exemplary finalgrism fabricated from method 40 is illustrated schematically in FIG. 6.

Step 42 of fabrication method 40 involves depositing a layer of patternmaterial 52 directly onto surface 33 of prism 25 followed by aphotolithography step 43. The deposition of the pattern material 52 ontothe prism defines the diffractive surface 33 of the prism.

Deposition step 42 can be effected by any number of depositiontechniques such as sputter coating, chemical vapour deposition, spincoating and thermal evaporation. It will be appreciated by those skilledin the art that the most appropriate technique for depositing thepattern material will be application specific and would not be limitedto the examples provided above. Similarly, the most appropriate patternmaterial will be application specific but in general will be some highrefractive index material such as silicon-nitride (Si₃N₄), TiO₂, HfO₂,amorphous silicon, high refractive index polymers such as spin-on-glass,photoresist, etc., Ta₂O₅ and metals.

The end result of photolithography step 43 is the production of adiffractive pattern in the pattern material to form a diffractiongrating 1, which is formed in situ directly onto the prism. By “insitu”, it is meant that steps 42 and 43 are performed directly onto asurface of prism 25, rather than fabricated on a separate substrate andsubsequently brought into contact (e.g. bonded) with prism 25.

Photolithography step 43 includes a number of sub-steps which aredescribed below.

The first sub-step 44 of the photolithography process 43 is to apply alayer of photoresist material 54 to the pattern material 52. Thephotoresist material 54 can be applied by any of the standard techniquessuch as spin coating, doctor blading, dip coating, spray coating, dryphotoresist etc. The position of photoresist material 54 is illustratedin FIG. 5.

The second sub-step 45 of the photolithography process 43 is to exposethe photoresist material 54, as illustrated in FIG. 5. The prism 25,including the pattern material 52 and photoresist material 54, is thenaligned to mask 55. The photoresist material 54 is then exposed to UVlight 56 through mask 55 such that only selected to regions 57 areexposed to the UV light 56. The mask 55 controls which regions of thephotoresist material 54 are exposed to the ultra-violet light 56.

The exposed regions 57 of photoresist are then removed through achemical development step at sub-step 46, uncovering the underlyingpattern material and producing a diffractive pattern in photoresistmaterial 54. The unexposed regions 58 of photoresist remain in place.Although sub-step 45 has been described with reference to a positivephotoresist, it will be appreciated that it can be equally achievedusing a negative photoresist.

Etching sub-step 47 transfers the diffractive pattern in the photoresistmaterial 54 to pattern material 52. The etching process involvesremoving uncovered pattern material while leaving the pattern materialunderlying the remaining photoresist 58 in place.

Washing sub-step 48 removes the remaining photoresist material 54,leaving the diffraction grating 1 on the prism 25 to define grism 23. Anexemplary side view of the resulting grism 23 is shown in FIG. 6.

An alternative method for performing sub-step 45 of the photolithographyprocess is illustrated schematically in FIG. 7 and explained withreference to FIGS. 8 and 9. Turning initially to FIG. 8, this methodmakes use of a diffraction pattern 80 created when UV light 81 passesthrough mask 82. The diffraction pattern 80 includes a repeating imageof the mask 82 at regular distances away from mask 82. The regulardistance at which the image repeats is given by the formula:

$z_{T} = \frac{2\; p^{2}}{\lambda}$

Where p is the period of mask 82 and λ is the wavelength of UV light 81.Another property of diffraction pattern 80 is that if it is integratedalong axis 83 over distance z_(T), it produces irradiance field 90 shownin FIG. 9. Irradiance field 90 is related to the physical arrangement ofthe elements of mask 82 but has a period 92 equal to half the period 94of mask 82. The inventors have identified that irradiance field 90 canbe used for photolithography and would in practice have a very largedepth of field. The large depth of field creates further relaxation onthe alignment requirements thereby further reducing manufacturing timesand costs. This alignment includes tilt alignment and surface flatnessof surface 33. Furthermore, for a given grism design, a mask with largerspatial features can be used since the irradiance field will have halfthe period of the mask. Masks with larger spatial features are typicallyless costly to produce, thereby further reducing manufacturing costs forthe grism. The requirements of the UV light source can similarly berelaxed allowing longer wavelengths to be used. For example, many priorart systems make use of deep UV light with a wavelength of 193 nmwhereas this method allows for the use of a wavelength of 365 nm for acomparable grism. In all, the second method may be able to reduce grismmanufacturing costs by around 85%.

Practically, irradiance field 90 can be used in photolithography bymoving the irradiance target, the photoresist, through diffractionpattern 80 of FIG. 8, along axis 83 for a distance equal to z_(T). Thismotion is represented by the arrow 60 shown in FIG. 7 where thediffraction pattern is created by passing light 56 through mask 59. Thismotion may be effected in a substantially continuous or stepwise mannerby any standard actuator. This motion selectively exposes certainsections of the photoresist to the UV light to define an exposurepattern, resembling the mask but with half the spatial period. Mask 59has twice the spatial period of mask 55 and results in the same grism 23shown in FIG. 6. The remaining sub-steps 46 to 48 are performed asdescribed above to produce grism 23.

This method, as previously mentioned, greatly relaxes alignmentrequirements increasing throughput of the manufacturing process andreducing the cost of the equipment required.

A further advantage is that exposure sub-step 45 permits multiplegratings to be exposed simultaneously. This is can be achieved byutilising a mask which is multiple times larger than diffraction surface33, thereby allowing multiple prisms to be aligned to a single mask andsimultaneously exposed. It will be appreciated that this can greatlyincrease the productivity of the fabrication process 40.

In addition to being applicable to the direct writing of gratings ontoprisms having non-parallel surfaces, the invention is also applicable towriting gratings onto optical elements having thicknesses greater than atypical wafer (less than about 1 mm). In particular, the above describedprocess is applicable for directly writing a diffraction grating onto asurface of a prism structure having a thickness of greater than 5 mm.That is, the input surface and the diffractive surface are separated bya distance of greater than 5 mm.

CONCLUSIONS

The single process technique of the present invention negates the costlyand time consuming process of patterning optical components. It alsoeliminates bonding of optical components—optical component bondingrequires additional equipment and reduces inconsistencies introduced bymulti-stage processes.

The present invention enables manufacture of grism assemblies withhigher performance and higher consistency, while reducing the cost by upto over 85%.

The equipment associated with the method of the present invention isabout 20 times cheaper than current grism fabrication methods, and ableto expose multiple components simultaneously with a single mask. Thismethod uses a larger wavelength light source (i-line @ 365 nm) comparedto DUV@193 nm, which in turn allows for cheaper masks. The equipment isalso not confined to exposing wafers, and can pattern prisms/odd shapedglass.

INTERPRETATION

It will be understood by one skilled in the art that the frequency andwavelength of a laser beam are connected by the equation:

Speed of light=wavelength*frequency.

As a consequence, when reference is made to frequency shifting,frequency converting, frequency broadening, different frequencies andsimilar terms, these are interchangeable with the corresponding termswavelength shifting, wavelength converting, wavelength broadening,different wavelengths and the like.

Throughout this specification, use of the term “element” is intended tomean either a single unitary component or a collection of componentsthat combine to perform a specific function or purpose.

Throughout the specification, the terms “manufacture” and “fabricate”,including their derivatives, should be interpreted as being synonyms.

Throughout the specification, the terms “optical” in the sense of“optical signal” and the like should be interpreted to include parts ofthe electromagnetic spectrum within and beyond the visible range,including but not limited to the infrared and ultraviolet ranges.

Unless specifically stated otherwise, as apparent from the followingdiscussions, it is appreciated that throughout the specificationdiscussions utilizing terms such as “processing,” “computing,”“calculating,” “determining”, analyzing” or the like, refer to theaction and/or processes of a computer or computing system, or similarelectronic computing device, that manipulate and/or transform datarepresented as physical, such as electronic, quantities into other datasimilarly represented as physical quantities.

In a similar manner, the term “controller” or “processor” may refer toany device or portion of a device that processes electronic data, e.g.,from registers and/or memory to transform that electronic data intoother electronic data that, e.g., may be stored in registers and/ormemory. A “computer” or a “computing machine” or a “computing platform”may include one or more processors.

Reference throughout this specification to “one embodiment”, “someembodiments” or “an embodiment” means that a particular feature,structure or characteristic described in connection with the embodimentis included in at least one embodiment of the present disclosure. Thus,appearances of the phrases “in one embodiment”, “in some embodiments” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures or characteristics may be combined inany suitable manner, as would be apparent to one of ordinary skill inthe art from this disclosure, in one or more embodiments.

As used herein, unless otherwise specified the use of the ordinaladjectives “first”, “second”, “third”, etc., to describe a commonobject, merely indicate that different instances of like objects arebeing referred to, and are not intended to imply that the objects sodescribed must be in a given sequence, either temporally, spatially, inranking, or in any other manner.

In the claims below and the description herein, any one of the termscomprising, comprised of or which comprises is an open term that meansincluding at least the elements/features that follow, but not excludingothers. Thus, the term comprising, when used in the claims, should notbe interpreted as being limitative to the means or elements or stepslisted thereafter. For example, the scope of the expression a devicecomprising A and B should not be limited to devices consisting only ofelements A and B. Any one of the terms including or which includes orthat includes as used herein is also an open term that also meansincluding at least the elements/features that follow the term, but notexcluding others. Thus, including is synonymous with and meanscomprising.

It should be appreciated that in the above description of exemplaryembodiments of the disclosure, various features of the disclosure aresometimes grouped to together in a single embodiment, Fig., ordescription thereof for the purpose of streamlining the disclosure andaiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claims require more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive aspects lie in less than all features of a singleforegoing disclosed embodiment. Thus, the claims following the DetailedDescription are hereby expressly incorporated into this DetailedDescription, with each claim standing on its own as a separateembodiment of this disclosure.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe disclosure, and form different embodiments, as would be understoodby those skilled in the art. For example, in the following claims, anyof the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the disclosure maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

Similarly, it is to be noticed that the term coupled, when used in theclaims, should not be interpreted as being limited to direct connectionsonly. The terms “coupled” and “connected,” along with their derivatives,may be used. It should be understood that these terms are not intendedas synonyms for each other. Thus, the scope of the expression a device Acoupled to a device B should not be limited to devices or systemswherein an output of device A is directly connected to an input ofdevice B. It means that there exists a path between an output of A andan input of B which may be a path including other devices or means.“Coupled” may mean that two or more elements are either in directphysical, electrical or optical contact, or that two or more elementsare not in direct contact with each other but yet still co-operate orinteract with each other.

Thus, while there has been described what are believed to be thepreferred embodiments of the disclosure, those skilled in the art willrecognize that other and further modifications may be made theretowithout departing from the spirit of the disclosure, and it is intendedto claim all such changes and modifications as fall within the scope ofthe disclosure. For example, any formulas given above are merelyrepresentative of procedures that may be used. Functionality may beadded or deleted from the block diagrams and operations may beinterchanged among functional blocks. Steps may be added or deleted tomethods described within the scope of the present disclosure.

We claim:
 1. A diffractive optical element including: an input surfaceconfigured to receive an input optical signal; a diffractive surfaceadapted to spatially disperse the input optical beam into a dispersedsignal; an output surface configured to output the dispersed signal;wherein the input surface and the diffractive surface are non-parallel;and wherein the diffractive surface is formed in situ by aphotolithographic technique.
 2. The diffractive optical elementaccording to claim 1 wherein the diffractive surface and output surfaceare non-parallel.
 3. The diffractive optical element according to claim1 wherein the input surface is the output surface.
 4. The diffractiveoptical element according to claim 1 wherein the optical element is atriangular prism.
 5. The diffractive optical element according to claim1 wherein the optical element is a trapezoid.
 6. The diffractive opticalelement according to claim 1 wherein the diffractive surface is formedby: depositing a pattern material layer of pattern material on thediffractive surface; applying a photoresist layer to the patternmaterial layer; creating a diffractive pattern in the photoresist layerusing a light source; and transferring the diffractive pattern in thephotoresist layer to the pattern material.
 7. The diffractive opticalelement according to claim 1 wherein the diffractive surface is formedby: depositing a photoresist layer on the diffractive surface; andcreating a diffractive pattern in the photoresist layer using a lightsource.
 8. The diffractive optical element according to claim 6 whereinthe pattern material is selected from one or more of Si3N4, TiO2, HfO2,amorphous silicon, high refractive index polymer, reflective metal andTa2O5.
 9. The diffractive optical element according to claim 8 whereinthe high refractive index polymer is spin-on-glass or photoresist. 10.The diffractive optical element according to claim 8 wherein thereflective metal is selected from chromium, gold, silver, aluminum ornickel.
 11. The diffractive optical element according to claim 6 whereinthe photoresist layer includes an anti-reflective coating and primer.12. The diffractive optical element according to claim 6 wherein thediffractive pattern in the photoresist layer is created by illuminatingthe photoresist layer through a photomask.
 13. The diffractive opticalelement according to claim 6 wherein the diffractive pattern in thephotoresist layer is created by illuminating the photoresist layerthrough a photo mask while moving the photoresist layer substantiallyperpendicular relative to the mask.
 14. The diffractive optical elementaccording to claim 13 wherein the movement is substantially continuous.15. The diffractive optical element according to claim 13 wherein thephotoresist layer is moved a distance of$z_{T} = \frac{2\; p^{2}}{\lambda}$ where p is the spatial period ofthe mask and A is the wavelength of the light source.
 16. Thediffractive optical element according to claim 14 wherein thephotoresist layer is moved a distance of$z_{T} = \frac{2\; p^{2}}{\lambda}$ where p is the spatial period ofthe mask and A is the wavelength of the light source.
 17. Thediffractive optical element according to claim 1 configured to operatein a wavelength selective switch.
 18. The diffractive optical elementaccording to claim 17 wherein the input optical signal includes aplurality of optical wavelength channels.
 19. The diffractive opticalelement according to claim 1 wherein the element includes a lens.
 20. Adiffractive optical element including: an input surface configured toreceive an input optical beam; a diffractive surface adapted tospatially disperse the input optical beam into a dispersed signal; anoutput surface configured to output the dispersed signal; wherein theoutput surface and the diffractive surface are non-parallel; and whereindiffractive surface is formed in situ by a photolithographic technique.21. The diffractive optical element according to claim 20 wherein theinput surface is the diffractive surface.
 22. A diffractive opticalelement including: an input surface configured to receive an inputoptical signal; a diffractive surface adapted to spatially disperse theinput optical beam into a dispersed signal; an output surface configuredto output the dispersed signal; wherein the input surface and thediffractive surface are separated by a distance of greater than 5 mm;and wherein the diffractive surface is formed in situ by aphotolithographic technique.
 23. The diffractive optical elementaccording to claim 22 wherein the element is non-wafer structure.
 24. Amethod of fabricating a diffractive optical element, the methodincluding the steps: depositing a pattern material layer of patternmaterial directly onto a first surface of a non-cuboid prism, whereinthe first surface is non parallel to either or both of an input surfaceconfigured to receive an input optical signal and an output surfaceconfigured to output a dispersed signal from the diffractive opticalelement; applying a photoresist layer to the pattern material layer;creating a diffractive pattern in the photoresist layer; andtransferring the diffractive pattern in the photoresist layer to thepattern material layer to define a diffractive surface on the firstsurface.
 25. A method of fabricating a diffractive optical element, themethod including the steps: depositing a photoresist layer directly ontoa first surface of a non-cuboid prism, wherein the first surface is nonparallel to either or both of an input surface configured to receive aninput optical signal and an output surface configured to output adispersed signal from the diffractive optical element; and creating adiffractive pattern in the photoresist layer to define a diffractionsurface on the first surface.